A riser tension protector and a coiled tubing lift frame assembly with over tension protection and back-up heave compensation are disclosed for use on floating drilling vessels such as drilling ships and semi-submersible drilling vessels.
As oil and gas offshore exploration and production operations are increasingly established in deeper waters, it has become more common for drilling activities to be performed from rigs that float on the surface of the water, such as drilling vessels or semi-submersible drilling rigs. Unlike fixed rigs or jack-up rigs, floating rigs are subject to wave motion, causing up-and down motion, which must be compensated for during drill, well completions, well testing, well interventions and other operations. Wave motion is of particular concern during “locked-to-bottom” operations (i.e., well completion, well testing and well intervention) where a completions workover riser or landing string (alternatively referred to as a ‘riser’) is physically connected to the subsea well at the seabed. It will be appreciated that, depending on the nature of the operations, the riser may be connected to a tubing hanger at the well-head, to a subsea tree or other infrastructure at the top of the well. Loss of heave compensation can lead to severe consequences.
Apart from the operational difficulties arising from the up-and-down motion of the floating rig, significant safety issues also arise, in particular the potential for the riser to fracture or buckle, resulting in loss of well containment and potential blowout. Indeed, safety standards in offshore operations demand that a heave compensation system be regarded as an essential component of a floating rig during locked-to-bottom operations.
Known heave compensation systems may be described as employing passive heave compensation or active heave compensation.
A simple passive heave compensator is a soft spring which effectively strokes in and out in response to string loads as the vessel heaves up and down while effectively holding constant tension on the string. Exemplary types of simple passive heave compensators are crown-mounted compensators or inline passive drill string compensators. Passive heave compensators employ hydraulic cylinders and associated gas accumulators to store and dissipate the energy as the vessel heaves up and down.
Active heave compensation differs from passive heave compensation by having an external control system with external inputs from motion reference units that actively tries to compensate for any movement at a specific point. Exemplary types of active heave compensation include active heave draw works which employ electric or hydraulic winch systems to raise and lower the top drive in response to the vessel motion.
Active-passive compensation systems consist of a primary passive compensation system with secondary actively driven hydraulic cylinders to reduce tension variations and improve efficiency. Two independent active and passive systems are generally not employed. The essential nature of the heave compensation function to a floating rig is such that safety standards also demand that they be designed such that no single component failure shall lead to overall failure of the system. They should also be “fail to safety” meaning that in the event of any predictable failures, the system defaults to a compensating state, which is the safest state during locked-to-bottom operations. While active heave draw works have numerous benefits for normal drilling operations, they fail to a “locked condition”, which is undesirable for well completions, well testing and well intervention operations. Passive compensation systems (e.g. crown mounted compensators) are also not immune to failures. Safe operations and industry standards require additional means of safety to be implemented in the system/equipment configuration. Additional means of safety may include a standard in-line tensioner or traditional compensated coiled tubing lift frame, design of a weak link in the riser/landing string, weaklink bails, limiting operation parameters to be within the stretch limit of the riser, and so forth.
Generally, these operating parameters place constraints on operators which have direct impact on productivity and efficiency. All these existing options have limitations. In the case of a standard inline tensioner or conventional compensated coiled tubing lift frame, there are concerns about the how the system behaves when run in series with the active heave draw works. In the case of the weak link in the riser and weaklink bails, they typically only provide protection in an over-tensioned case and once broken, they provide no support to the landing string thereafter. In the case of limiting operating parameters to within the stretch of the riser, this can impose considerable downtime during offshore operations.
It would be advantageous to provide an inline tensioner or a back-up compensator which can be fully locked under normal operating loads so that it did not interfere with the operations of the primary compensator, but which is capable of automatic and rapid activation to provide compensation if axial load on the riser exceeds normal operation limits, which may occur in event of failure of the rig's primary compensator.
It would also be desirable to have an inline tensioner or a back-up compensator that may be actuated without the need for control valves to control fluid flow through fluid lines. Control valves add complexity into the back-up compensator, introducing additional failure points and restricting the actuation speed of the back-up compensator.
There is therefore a need for an alternative or improved heave compensation apparatus which may operate as a back-up to the rig's drill string compensator in the event of failure or disablement of the rig's drill string compensator.
There is also a need for an improved heave compensation apparatus which can be used as a lift frame for the installation of intervention pressure control equipment (i.e. coiled tubing or wireline equipment) during well testing/well intervention work, as those components are installed in the congested space of the drilling derrick.
The above references to background art do not constitute an admission that the art forms a part of the common general knowledge of a person of ordinary skill in the art. The above references are also not intended to limit the application of the heave compensation and tensioning apparatus as disclosed herein.